All the watermelon we eat today is a product of domestication, one of the earliest forms of modifying the genes in a crop.

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The first genetic modifications, and broadest sense of the term GMO, has been happening throughout the history of farming.

By picking certain traits, such as bigger fruit, more seeds, and better color, farmers are inherently selecting the genes for certain crops. That’s how the watermelon we eat became so different from versions from the 1600s.

About 300 years ago, farmers and scientists started cross-breeding, or fertilizing new types of plants using two parent plants that wouldn't normally come together in nature. That’s how we get things like potatoes as we know them and many kinds of berries, Bruce Chassy, a professor emeritus of food safety and nutritional science at the University of Illinois, told Business Insider.

Corn was domesticated hundreds of years ago as well, but the stuff we eat today has been modified further — with genes from living things that are not plants.

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Corn, the most widely grown crop in the US, is currently modified in two key ways: either by adding genes from Bacillus thuringiensis bacteria, known as Bt, to make it resistant to corn-loving insects, or genes from Agrobacterium, which makes it resistant to the weedkiller Roundup.

In the 1990s, papayas in Hawaii faced extinction without help from GMO technology.

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Papayas in Hawaii were facing destruction from the Ringspot virus, a disease transmitted to the fruit by insects. To fix the problem, scientists added a harmless gene from the virus into the papaya's DNA, giving papayas immunity to the virus. Today, most papayas are produced in Hawaii, though some come from Texas, California, and Florida as well.

Soybeans were genetically modified around the same time as corn and other crops to resist herbicides like RoundUp.

Soy seems to be everywhere these days: From edamame and soy sauce to soy lecithin, which is used to help thicken things like salad dressing, it's almost unavoidable. Soybeans were genetically modified around the same time as corn and other crops to be resistant to herbicides.

As of 2010, 90% of US canola oil was genetically modified.

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Rapeseed, the plant that's used to make canola oil, is another crop that's genetically modified to be used in combination with herbicides that kill weeds but leave the plants untouched.

Canola oil is used in cooking exactly like olive oil, so it shows up in a lot of prepared meals you buy at the grocery store, like potato salad or coleslaw. It's also used to deep-fry a lot of foods, including McDonald's french fries.

Shortly after its approval, officials predicted that about 50% of alfalfa made in the US will be genetically modified, according to the Wall Street Journal.

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Unless you're really into alfalfa sprouts, you're not going to run into much genetically mofidied alfalfa during the course of your day. But it is used to feed livestock, like these cows chowing down on a mix of alfalfa and hay.

Alfalfa, like many other crops, was genetically engineered to resist glyphosate, which is a chemical used to kill weeds, and got FDA approval in 2011.